1. Field of the Invention
This invention relates to a full automatic or semiautomatic focusing camera using an interchangeable photo taking lens and more particularly to a camera of the kind arranged to project a focus point detecting light flux or beam through the photo taking lens and to detect through the photo taking lens a light flux or beam reflected by an object.
2. Description of the Prior Art
The focus detecting device of the kind arranged to project a light flux or beam on an object to be photographed and to detect a focus point by receiving a reflection light flux from the object has been known as advantageous in that: The system of this kind is capable of detecting the focus point even in the event of a dark object or an object having a low degree of contrast. In addition, it gives no parallax in an in-focus state. Further, it is capable of preventing the degradation of focus detection accuracy due to the surface reflection of the photo taking lens.
FIG. 1 of the accompanying drawings shows by way of example the optical system of the light projecting type focus detecting device of the kind mentioned above. The illustration includes a photo taking lens which is attachable to and detachable from a camera body; a light emitting element 2 disposed within the camera body; a light receiving lens 3; and a light receiving photo-sensitive element 4. Both the light receiving lens 3 and the photo-sensitive element 4 are disposed on the camera body. The light emitting element 2 is arranged on the optical axis O of the photo taking lens 1. A beam of light emitted from the light emitting element 2 is projected via the photo taking lens 1 on the surface of an object to be photographed. The projection light flux or beam thus obtained has the optical axis of the photo taking lens 1 at the center thereof irrespective of the focal point of the lens 1 and the object distance. A resultant reflection light flux from the object comes through the light receiving lens 3 to be imaged on the photo-sensitive element 4. In accordance with the principle of triangulation, the displacement D of the center position of the received light flux imaged on the photo-sensitive element 4 can be expressed as follows from the object distance R, a base length L which is a distance between the optical axis O of the projected light flux and the optical axis P of the received light flux and a distance S between the light receiving lens 3 and the photo-sensitive element 4: ##EQU1##
In other words, the displacement D of an imaging position on the photo-sensitive element 4 is determined by the object distance. Conversely, the object distance R can be determined by obtaining the displacement D of the imaging position of the received light flux on the photosensitive element 4. Then, the focus is adjustable by controlling the focal point of the photo taking lens 1 according to the object distance R thus obtained.
Meanwhile, in the camera of the kind using an interchangeable photo taking lens, the optical axis of the photo taking lens sometimes deviates from that of the camera and thus becomes eccentric when the former is mounted on the latter.
FIG. 2 schematically shows the focusing optical system of FIG. 1 as in a state of having the photo taking lens 1 mounted with some eccentricity .DELTA.Y. Assuming that the focal length of the lens 1 is f, the imaging displacement D of the received light flux on the photo-sensitive element 4 varies to a displacement D' and there arises an error .DELTA.D as apparent from FIG. 2. The degree of this error .DELTA.D can be expressed from a light projection angle error .DELTA..theta. (in radian)=.DELTA.Y/f as shown below: ##EQU2##
With respect to a focusing error .DELTA.x which results from this eccentricity .DELTA.Y, the deviating degree .DELTA.D of the light flux and an object distance error .DELTA.R can be expressed as follows by differentiating both members of Formula (1): ##EQU3##
Further, from the image formation formula, a relation between the object distance error .DELTA.R and the focusing error .DELTA.x can be expressed as follows: ##EQU4##
A relation between the eccentricity .DELTA.Y of the photo taking lens and the focusing error A.DELTA. can be expressed from Formulas (2), (3) and (4) as shown below: ##EQU5## as will be apparent from Formula (5) above, in the event of a long focal length f of the photo taking lens 1 preventing arrangement to have a large base length L, a large focusing error results even from a slight eccentricity of the lens 1.
For example, assuming that the photo taking lens 1 has a focal length f=100 mm and a brightness (or aperture) of F/2.8 and that the base length L is 30 mm, in order to reduce the degree of blur due to the focusing error to a value less than an allowable diameter of circle of confusion .phi.=0.035 mm, the value of an allowable eccentricity .DELTA.Y of the photo taking lens can be obtained from Formula (5) as .DELTA.Y=30/100.times.(2.8.times.0.035).div.0.03. The allowable eccentricity .DELTA.Y thus must be set at a value less than 0.03. However, such an allowable eccentricity is considered too severe for a camera of the kind arranged to have an interchangeable lens mounted thereon because of the eccentricity due to the play allowed for mounting.
Thus, with the optical axis of the photo taking lens deviating from the optical axis of the camera body, there have been the fear of degradation of focusing accuracy resulting from that the reflection light flux or beam from the object is received by the photo-sensitive element with some error.
Meanwhile, Japanese Laid-Open Patent Application No. SHO 57-73709 (corresponding to U.S. Pat. No. 4,394,077) has disclosed a camera equipped with a light projecting type focus detecting device. According to the disclosure, a light receiver is arranged within a camera body while a light projector is disposed in a lens barrel. However, since the light receiver which is arranged within the camera body is disposed behind the photo taking lens, the invention disclosed seems to have paid no heed to the above-stated problem of eccentricity.